This invention relates to a process and adsorbents for selective adsorption of a gas component, and particularly, selective adsorption of nitrogen.
The separation of air for the production of nitrogen and oxygen is a very important operation in the chemical processing industry. Historically, this separation has been done predominately by cryogenic distillation; though, as adsorption systems have become more efficient and new, more effective sorbents have been synthesized, separation by adsorption processes (e.g., pressure swing adsorption (PSA), and vacuum swing adsorption (VSA)) have become increasingly competitive and are already favorable for small-to-medium scale operations. Currently, approximately 20% of air separations are accomplished using adsorption technologies.
Since their introduction in the late 1950""s, synthetic zeolites have been used in numerous applications such as catalysis, ion exchange, drying, and separation by selective adsorption. In the separation of air, zeolites of type A and X have typically been used. (See U.S. Pat. No. 5,551,257, Jain). The A and X type zeolites are composed of silica and alumina tetrahedra which are joined together to form the truncated octahedral or sodalite structure. These sodalite units are connected with tertiary units to form the structured zeolite unit cell. While the SiO2 groups are electroneutral, the A102 groups are not, and thus introduce a negative charge to the structure which is offset by the presence of a charge compensating, non-framework cation (e.g., Na+, Li+, Ca2+). Type X zeolites contain between 77 and 96 Al per unit cell. The unit cell, including cation sites, for the X zeolite is shown in FIG. 1.
The extra-framework cations in the zeolite are largely responsible for the nitrogen selectivity of these materials. These zeolites adsorb nitrogen preferentially to oxygen (usually at a ratio of about 4:1) due primarily a to the interactions between the charge compensating cations of the zeolite and the quadruple moment of the adsorbing gas (N2 or O2). The quadruple moment of N2 is approximately four times that of 02. Because the extra-framework cations so significantly influence the adsorption properties of the zeolites, numerous attempts have been made to optimize these properties by (1) increasing the number of cation sites (the cation exchange capacity, CEC) by creating zeolites with high aluminum content, and (2) by synthesizing zeolites containing various alkaline, alkaline earth, and combinations of these cations.
Low silica X-type zeolite (LSX) is known. This material is an aluminum saturated X-type zeolite with a silica-to-alumina ratio of 2.0 (or Si/Al=1.0). Commercial X-zeolite, which is typically available as the Na+ form (known commercially as 13X), is not aluminum saturated and contains 86 aluminum atoms per unit cell, while the low silica X zeolite contains 96 aluminum atoms per unit cell.
Li+ is among the strongest cations, with respect to its interaction with N2, its use was greatly increased with two recent advances. First, it was found that Li+ ion-exchange in X-type zeolite must exceed an approximate 70% threshold before the Li+ has any affect on the adsorption properties of the material (U.S. Pat. No. 4,859,217, Chao). Second, a significant increase in the N2 adsorption capacity was seen in Li+ ion exchanged low silica X-type zeolites over that of the typical commercial zeolites (Si/Al=1.25). Because of these advances, Lixe2x80x94X (Si/Al=1.0) is now the best sorbent in industrial use for separation of air by adsorption processes (U.S. Pat. No. 5,268,023, Kirner; U.S. Pat. No. 5,554,208, Mullhaupt).
Sicar et al., U.S. Pat. No. 4,557,736 and Coe et al., U.S. Pat. No. 4,481,018 reported the use of a binary exchanged X-zeolite having lithium and calcium and/or lithium and strontium ions in a ratio of 5% to 50% calcium and/or strontium and 50% to 95% lithium. This zeolite provided for enhanced nitrogen adsorption over those of the Naxe2x80x94X, Lixe2x80x94X and Caxe2x80x94X zeolites. They also reported the use of mixed ion-exchanged A and X zeolites with lithium and an alkaline earth metal (e.g., Ca2+, Sr2+). In this case the zeolite contained lithium and the alkaline earth cations in a mixture of 10% to 70% alkaline earth and 30% to 90% lithium. These mixed cation zeolites provide good adsorption capacity and good thermal stability. However, the cost of separation still remains high. Therefore, there remains the need for improved methods and adsorbents to effectively and economically separate nitrogen from a gaseous mixture.
The invention provides new methods for separating nitrogen from a mixture. The invention provides adsorbents specifically for accomplishing. nitrogen separation. The adsorbents and separation methods are particularly useful for the selective adsorption of nitrogen from air. In one aspect, the adsorbent comprises an ion exchange zeolite X and preferably zeolite LSX (low silica zeolite X). The zeolite is most preferably a lithium-based zeolite. Further, the zeolite has exchangeable cationic sites, with silver cation or copper cation occupying at least some of the exchangeable cationic sites. The presence of the silver cation or copper cation at any of the sites will provide an improvement over the non-exchanged zeolite. Therefore, the minimum amount of silver cation or copper cation is greater than zero. The inclusion of silver cation and/or copper cation at the exchangeable cationic sites provides such an improvement in strength of adsorption of nitrogen, that any amount is helpful. However, consideration is given to the strength of such adsorbent capacity when optimizing the amount, in view of subsequent desorption. Since Ag+ and Cu+ strongly hold nitrogen, it is desirable that the amount of such cation be up to about 20% of the exchangeable cationic sites. It is preferred that the silver or copper cation occupy about 10% of the exchangeable cationic sites. Such optimization leads to a good balance between strength of adsorption and facilitating subsequent desorption. Therefore, it is evident that not all of the ion exchangeable cationic sites of the zeolite will contain copper or silver and preferably less than half of such sites will be so exchanged.
Zeolites are known and have been used as adsorbents due to their selectivity. Crystalline zeolite Y, zeolite A, and zeolite X are examples and are described in U.S. Pat. Nos. 3,130,007; 2,882,243; 3,992,471; and 2,882,244; each of which is incorporated by reference in its entirety. Type 5A zeolite, and type 13X zeolite are described for nitrogen adsorption in U.S. Pat. No. 5,551,257, also incorporated herein by reference in its entirety. Low silica X zeolite (LSX) having Si/Al ratio less than or equal to 1.25, desirably less than or equal to 1.2, and preferably about 1, is described in U.S. Pat. No. 5,268,023. Each of the aforementioned patents is incorporated herein by reference in its entirety. Consistent with the features described in these patents, zeolite characteristics are also described in the reference book entitled xe2x80x9cGas Separation by Adsorption Processesxe2x80x9d by R. T. Yang (1987 Butterworth Publishers). To the extent that zeolite characteristics are pertinent to the present invention, they will be described further hereinbelow.
In the practice of the invention, the important characteristic desired is imparted by the presence of silver and/or copper cation in a zeolite which has been previously exchanged to provide a lithium X zeolite or a lithium LSX zeolite. The desirable X zeolite has a silicon to aluminum ratio (Si/Al) of about 1 to about 1.3. The more desirable lithium LSX has the preferred silicon to aluminum ratio of 1.0. Therefore, the adsorbents of the invention are essentially silver or copper ion exchanged Li+ zeolites. The presence of the silver cation or the presence of the copper cation in combination with the lithium cation provides the desired characteristic for improved nitrogen adsorption. However, the zeolite may also include minor amounts of other commonly found cations which occur in zeolite including, but not limited to besides lithium, potassium, sodium, rubidium, caesium, and mixtures thereof which are alkali metal cations; and alkaline earth metal cations beryllium, magnesium, calcium, strontium, barium, and mixtures thereof. The presence of a trivalent cation is also possible, however, such is not preferred in order to provide available sites for occupancy by the preferred silver, copper, and lithium.
In another aspect, the adsorbents of the invention are used in a method for separating nitrogen from a gaseous air mixture, by accomplishing adsorption at a first select pressure and temperature and then accomplishing release or desorption by changing at least one of the pressure and temperature. Preferential adsorption of nitrogen is preferably achieved by pressure swing adsorption. Conveniently, this may be carried out and is preferably carried out at about ambient room temperature conditions. Therefore, special temperature treatment is not required. In the pressure swing process, the preferred range for adsorption is about 1 to about 10 atmospheres, and the preferred range for desorption is about 0.2 atmospheres to about 1 atmosphere.
In the process for preparing the zeolites of the invention, first lithium zeolites are prepared by ion exchange using lithium chloride. Then these lithium-zeolites were used to prepare LixAgy-zeolites and LixCuy-zeolites. For convenience, these will be referred to as mixed cation zeolites containing lithium, and transition metal capable of a +1 valence state (targeted metal ion). The preparation of the LixAgy-zeolites is exemplary and is accomplished by ion exchange of a Li-zeolite, prepared as described earlier, with a solution of silver nitrate. The copper ion exchange is accomplished in a comparable manner. Ion exchange of zeolite is easily accomplished by mixing the zeolite in an aqueous solution of metal salt. The metal of the salt is the metal to be exchanged into the cationic site. The concentration of the solution is varied according to the desired level of ion exchange. The ion exchanged zeolite is then removed by filtration from the aqueous solution and washed free of the soluble salts. The Cu-zeolites of the invention are prepared by ion exchanging with a copper salt solution preferably copper chloride or copper nitrate, followed by reduction of any copper +2 to copper +1.
After the incorporation of the targeted metal ion, the mixed cation material is dried at room temperature and atmospheric conditions. Dehydration in vacuo may follow later, and prior to use and/or analysis. Zeolites have a strong affinity for water; and some molecules are tenaciously held. The presence of water in the zeolite affects measurement. In the process of the present invention, specific conditions for heat treatment are used beyond the treatment required for mere dehydration. In the present invention, specific heat treatment is used to optimize performance of the mixed cation zeolite of the invention. The heat treatment, after ion exchange, of the mixed cation zeolite is above a minimum temperature of approximately 400xc2x0 C. A temperature of 400xc2x0 C. or greater is required in order to form crystal clusters of silver and/or copper. The upper limit to the heat treat temperature is 700xc2x0 C.; and preferably is below 700xc2x0 C., as this is determined to be the point at which destruction of the zeolite itself occurs. The heat treatment is able to be accomplished in air, in vacuum, in inert atmosphere such as argon, nitrogen, or in reducing atmosphere. Desirably, the heat treatment is in a non-oxidizing atmosphere such as in vacuo, in inert atmosphere, or reducing atmosphere. An air atmosphere is less desirable. The non-oxidizing atmosphere is selected to produce partially metallic clusters, and provide the cluster formation and character of the zeolite product described herein. Here inert means inert with respect to the metal ions, cluster formation and character of the zeolite. Thus, the atmosphere needs to be unreactive with the zeolite, and not interfere with formation of desired ion clusters. Treatment temperatures on the order of 20 to 30 minutes are thought to be a minimum. There is no real maximum to the duration of heat treatment time and such time has been extended to 5 hours without any difficulty. Typical heat treatment time varies from about 1 to about 4 hours; and more typically 1 to 2 hours.
The preferred lithium content of the zeolite is such that, of the available cationic sites, 70% or more and preferably 80% or more of such sites contain lithium. It is preferred that the proportion of cationic sites occupied by the silver and/or copper be up to about 10%, although up to about 20% is workable as described earlier. Compositions as described hereinbelow were prepared and found to be operable for a variety of ranges including 0.5 to 5% of the cationic sites occupied by silver; over 88% of cationic sites occupied by lithium; and with other alkali and alkaline earth metals constituting the balance. The compositions contain the aforesaid metallic clusters where the metal (M), copper or silver, is desirably partially metallic. This is exemplified by clusters of n number of metal atoms collectively having a charge represented by nxe2x88x921. This is expressed as Mn(nxe2x88x921) where n is 2 or more, and examples are Ag32+ and Ag65+.
The invention provides substantial advantages over conventional methods for separating nitrogen from an air mixture due to the effective and economical processes and adsorbents provided by the invention.
Objects, features, and advantages of the invention include an improved method for separating nitrogen from a gaseous mixture, and particularly for separating nitrogen from air. Another object is to provide new adsorbents used in such new separation method.
These and other objects, features, and advantages will become apparent from the following description of the preferred embodiment, claims, and accompanying drawings.